Photocurable prepolymers and compositions are well known in the art for forming printing plates and other photosensitive or radiation sensitive articles. In the field of radiation sensitive flexographic printing plates, the plates typically comprise a support and a photosensitive surface or layer from a photocurable composition. Additional layers or surfaces on the plate include slip and release films to protect the photosensitive surface. Prior to processing the plate, the additional layers are removed, and the photosensitive surface is exposed to radiation in an imagewise fashion. The unexposed areas of the surface are then removed in developer baths.
Removal of unexposed surfaces comprising solid photocurable compositions such as those disclosed in U.S. Pat. No. 2,760,863 require the use of developer baths comprising environmentally unsafe, organic solvents such as tetrachloroethylene, 2-butanone, benzene, toluene, xylene, trichloroethane and solvent mixtures such as tetrachloroethylene/n-butanol However, due to the toxicity, high volatility and low flash point, their use gives rise to hazardous conditions and creates pollution problems. Thus, recently there has been a strong interest in the field to develop photosensitive layers in non-organic solvent developing solutions, e.g. aqueous, surfactant-aqueous or alkaline-aqueous solutions. However, the compositions resulting from recent attempts to achieve aqueous developable plates demonstrate deficiencies in mechanical properties, e.g. flexibility. See European Application 261,910.
For instance, in addition to possessing an aqueous developable photosensitive surface, a flexographic printing plate must have sufficient flexibility to wrap around a printing cylinder, yet be strong enough to withstand the rigors experienced during typical printing processes. Further, the printing plate should possess a low hardness or softness to facilitate ink transfer during printing.
Previous aqueous developable compositions have not possessed all the desirable features such as flexibility, softness and solvent resistance to inks typically used in printing. For example, U.S. Pat. No. 4,023,973 describes a photosensitive composition comprising a maleic anhydride adduct of a 1,2-polybutadiene. However, because the 1,2 content of this material is very high, i.e. 70% or more, this composition has an undesirably high rubber hardness.
Furthermore, other water-developable photosensitive compositions which contain as the main component a high molecular weight polymer such as polyvinyl alcohol, cellulose, polyethylene oxide, or the like, are insufficient in flexibility and possess a high degree of rubber hardness and hence are unsuitable for use in flexographic printing plates.
Finally, it is also important that the photosensitive surface of the printing plate be dimensionally stable during storage. For example, some compositions used for making plates have shown inferior stability properties when used in solid flexographic printing plates in that the compositions become tacky and pasty during storage. Those inferior properties have been attributed to the low molecular weight of the polymers used to prepare the printing plates. See U.S. Pat. No. 4,762,892 to Koch et al. and discussion of low molecular weight polymers disclosed in Japanese Kokoku 57-23693.
Carboxyl-modified urethane prepolymers are known, and the aqueous dispersibility of several has been studied. One type of aqueous dispersible composition known in the art is a carboxylated urethane. The following disclose representative examples. A paper by R. C. Hire and S. C. Cohen, "Carboxyl Modified Polyols in Urethane Applications", in Proceedings of the SPI-32d Annual Conference, Oct. 1-8, pp. 118-125 (1989), discloses the preparation and certain reactions of carboxyl modified polyols ("CMP"). For example, the starting CMP (which can be like the base polymer of the instant invention) is treated with MDI to provide an isocyanate terminated prepolymer. The latter is then treated with a diol to form a urethane terminated prepolymer. Thus, "Slide 8" of the paper shows various ratios of starting polyol, diisocyanate, and butane diol. Slide 21 shows reacting MDI-capped base polymer with dimethylolpropionic acid (DMPA) to form ionomer, followed by chain extension with triethylamine or ethylene diamine. Acrylate-diol chain extenders are not disclosed, nor is chain-termination with an acrylate. In no case is there sufficient diisocyanate to give a final chain extended polymer. The final product is either terminated with --OH at both ends or with --OH at one end and --NCO at the other, or with --NH.sub.2 (when extended with a polyamine). My process differs inter alia in that I use an excess of diisocyanate, so that the final chain-extended prepolymer is totally capped with --NCO. The latter consequence is necessary in my process, since in my final step, I add a hydroxy acrylate, the hydroxyl of which must react with the prepolymer --NCO's in order to form the final acrylate terminated photopolymer.
R. C. Hire also offered a similar paper, "Carboxyl Modified Polyols in Water-Borne Urethane Coatings," at the Water-Borne and Higher-Solids Coatings Symposium, Feb. 1-3, 1989, in New Orleans; pp. 357-358 disclose end-capping the carboxyl-grafted base polymer with a diisocyanate, followed by reaction with DMPA. This ionomer is capped with a polyamine and is then ready for water dispersion.
The two papers above are based on research at the Olin Corporation, of New Haven, Conn.
U.S. Pat. No. 4,460,738, Jul. 17, 1988, to Frentzel et al (assigned to Olin Corporation) discloses substantially the same process described in the above papers, including treating the acid grafted polyol with, e.g., MDI, and chain-extending with, e.g., butane diol. Acrylate diol chain extenders are not disclosed. The grafted --COOH reacts at least partially with --NCO to form amide. In Col. 7, lines 22-25, the ratio of --NCO groups to --OH groups in the reactants (i.e., in the acid-grafted polyols) is stated as 0.5-5:1. This will of course cap all --OH groups in the polyol, including at least partial amide formation. The product is "neutralized", e.g., with a diol. The urethane prepolymer product can be chain extended with a diamine, a diol or mixtures (Col. 7, lines 54-55). Neither dimethylolpropionic acid (DMPA) nor a (meth)acrylate(s) containing diol is mentioned as chain extenders. The chains so extended are apparently not terminated with --NCO. Example 43 of the patent discloses reacting together acid-grafted polyol, diisocyanate, and dipropylene glycol (as hardener). Again, my process differs from that of the patent, inter alia, in that the diol chain extenders used contain carboxylic acid groups optionally with photoactive (meth)acrylate groups. In my process, after the polyol-diisocyanate reaction is complete, a chain-extending diol such as DMPA is added, so that the resulting chain-extended prepolymer terminates first with hydroxyls, and then on further reaction with the excess diisocyanate present, the result is a diol chain-extended --NCO capped prepolymer. The latter class of compounds is apparently not taught in the referenced literature.
U.S. Pat. No. 4,590,255 O'Connor and Frentzel (assigned to Olin Corporation) has a disclosure similar to that in their prior U.S. Pat. No. 4,460,738. However, there is no mention of (meth)acrylate(s) containing diol chain extenders, DMPA, or hydroxyalkylmethacrylate. The final product is neither UV-curable nor water-developable.
A paper by N. Barskby and R. M. Gerkin, "Acid Grafted Polyethers: Reactive Intermediates for Use in Low VOC Coatings Applications," Proceedings of the Southern Society for Coatings Technology, 17th Annual Water-Borne and High-Solids Coatings Symposium, Feb. 21-23, 1990, discloses (in the section, "Water-Borne Polyurethanes") preparation of waterborne polyurethanes by reacting together diisocyanate, acid-grafted polyether polyol, and chain extender at 80.degree. C. to form an isocyanate-terminated prepolymer. The chain-extender in this section is 1,4-butanediol (Table 12, Column 3). Amine is added to form a salt, and then the result is reacted with an amine chain extender or cross-linker. A water dispersion is made up and the result further chain extended, again apparently with an amine. In Table 12, Formulation 3 reacts together 100g (0.1 mole) acid-grafted polyoxypropylene diol ("PWB-1200:), 89.9 g (0.4 moles) isophorone diisocyanate (IPDI), and 24.3 g (0.27 moles) 1,4-butanediol (BDO). Since the 0.1 mole PWB1200 reacts with 0.2 mole IPDI, 0.2 mole IPDI is left for chain extension reactions, 1:1, with BDO. The BDO is thus in excess and the chain will terminate with --OH, not --NCO.
A technical brochure, "UCarmod", available from Union Carbide Corporation (1990) discloses that "Ucarmod acid-grafted polyether PWB-1200" (as used in the below examples) is "specially designed for use in water-dispersible polyurethane coatings and adhesives", but no directions are given.
Other references include:
Knopf, J. K. and Drake, K.; U.S. Pat. No. 4,528,334, "Carboxylated Poly(oxyalkylenes)", July 1985. This patent describes the preparation of acid-grafted polyether polyols that can be used as the base polymer in the instant invention.
Japanese Application 02 46460, Tomita, A. and Kawahara, K. "Photosensitive resin compositions for flexographic plates." February 1990 (Toyobo). The compositions disclosed in this reference lack carboxylic acid groups on the base polymer (a polyether polyol). Furthermore, acrylates, though present, are merely used as diluents (or fillers) and are not attached on the polyurethane polymer.
Satomi, H. et al., Japanese HEI Application 2-4255, "Light Sensitive Resin Composition", January 1990 (Toyobo). The base polymer disclosed therein to prepare a polyurethane is not a poly(oxyalkylene) and lacks carboxylic acid.
Gersdorf, J. and Kroggel, M.; European Patent Applications 374706 and 374707. June 1990 (Hoechst, A. G.) "Photocurable elastomer mixture containing esterified graft polyurethane, unsaturated monomer and initiator, useful for printing plate and photoresist production." The compositions disclosed in this patent lack carboxylic acid groups and acrylate groups on polyurethane. Acrylates are used as diluent (or fillers). Water dispersibility is obtained by grafting vinyl carboxylates on the polyurethane and then converting to --OH groups.
Gersdorf, J. et al. European Application 0351628. July 1989 (Hoechst AG.) "Photosetting elastomer mixture and recording material obtained from it for the production of relief printing plates." The polyurethane lacks carboxylic acid and acrylate groups. Acrylates are used as diluents (or fillers). Water dispersibility is obtained from --OH groups pendant on polyurethanes. In my invention water-dispersibility is obtained from carboxylic acid groups pendant on polyurethanes.
Joerg, K. and Zertani, R. West German Patent 3841025. June 1990 (Hoechst AG.) "Radiation polymerizable mixture containing saturated polyurethane-urea binder unsaturated compound and photopolymerization initiator, e.g., for photoresist or printing plate." The --COOH containing diols used for reaction with diisocyanates are not polymers. The compositions disclosed in this reference lack acrylates on the polyurethane. Acrylates are used as diluents or fillers to obtain UV curability. In one embodiment a polyether diol is used as a base polymer, but the compound is free from acid groups.
Japanese Patent Application 02077748. March 1990. (Fuji Photo Film KK.). "Photosensitive composition used for printing plate, etc., based on polyurethane resin that is insoluble in water but soluble in aqueous alkaline solution." In this reference, the polyurethane has an imide group, --CO--NH--CO that forms a ring with the polyurethane main chain. The polymer disclosed in this reference lacks both carboxylic acid groups and acrylate groups on polyurethane.
Aoai, T. et al. U.S. Pat. No. 4,950,582, "Light-sensitive Composition," March 1988 (Fuji Photo Film). This reference describes polyurethane made from base polymer that has N-sulfonyl amido groups. The base polymer lacks carboxylic acid groups and the final polyurethane lacks both carboxylic acid groups and acrylates.
In summary, the above compositions lack a major feature of my invention. In particular, this feature is a combination of three elements:
(1) the base polymer which can be a conventional polyether diol (herein base polymer Ia); an acid-grafted polyether diol (herein base polymer I) or a mixture of acid grafted polyether diol with a conventional polyether diol;
(2) the chain that extends the base polymer, which is a mixture of diols containing carboxylic acid groups and optionally photoactive (meth)acrylate groups; and
(3) the hydroxyalkyl(meth)acrylate, which is used to terminate the chain ends, and is optional when photoactive (meth)acrylate groups are used in (2).